Thermo-optically tunable laser system

ABSTRACT

A tunable laser has a solid state laser medium with optical gain region and generates coherent radiation through a facet. A lens collects the coherent radiation and generates a collimated light beam. An external cavity includes a reflective surface and an optical filter, the reflective surface reflecting the collimated beam back to the lens and laser medium, the optical filter positioned between the reflective surface and the lens and having two surfaces and a thermally tunable optical transmission band within the optical gain region of the laser medium. The optical filter (1) transmits a predominant portion of the collimated beam at a desired wavelength of operation, and (2) specularly reflects a remaining portion of the collimated beam from each surface, the collimated beam being incident on the optical filter such that the reflected collimated beams propagate at a non-zero angle with respect to the incident collimated beam.

SUMMARY

Tunable optical components capable of varying optical transmission as afunction of an input parameter have many applications in optics. Oneexample is an air gap etalon with piezoelectric transducers that canvary its air gap and thus optical pass band as a function of anelectrical signal. Another example is a silicon etalon with a passbandthat changes as a function of etalon temperature due to the temperaturedependence of the index of refraction of silicon.

Quantum cascade lasers (QCLs) are unipolar solid state lasers thatachieve lasing at infrared wavelengths, including wavelengths in therange between 3 and 13 um that are of considerable interest in molecularspectroscopy. QCLs are available in different design configurations,including distributed feedback (DFB), and Fabry-Perot (FP) lasers. Ofparticular interest in many spectroscopy applications are tunablelasers, that is lasers that through adjustment of an operating parametercan achieve lasing at different wavelengths. An adjustable parameter mayinclude the operating temperature of the laser or the laser drivecurrent. Lasers in DFB or FP configurations typically have very limitedwavelength tuning ranges through such adjustments, as in both of theseconfigurations the laser cavity is confined to the QCL lasing medium.However, external cavity devices that extend the laser cavity outsidethe QCL medium do not have the same limitation. Through the use ofgratings or other wavelength selection components, relatively widetuning ranges, in excess of 20% of a center wavelength, can be achievedif supported by the gain medium of the laser. Capability for a broadgain region is one feature of the QCL that is particularly attractive inmolecular spectroscopy, as it helps to provide improved backgrounddiscrimination as well as the ability to detect the absorption lines ofmultiple materials (gases, liquids and solids) of interest with a singlelaser. Thus, methods of tuning the laser to multiple wavelengths usingwavelength selection components is of interest, particularly those thatare amenable to low cost manufacturing and environmental stability inapplications that encompass medical, industrial, environmental,military, and biotechnology markets. The following disclosure describesmultiple embodiments of a thermo-optic tunable filter and multipleembodiments of thermo-optic tunable laser comprising one or morethermo-optic tunable filters as wavelength selection devices.

In one embodiment, an external cavity laser comprises a quantum cascadelaser (QCL) and one or more thermo-optically tunable (TOT) filterswithin an external cavity of the laser. The filters are fabricated usingmicromachining (MEMS) process techniques and comprise one or moreoptical cavities containing at least one thermo-optic material anddistributed Bragg reflector (DBR) mirrors made from at least twomaterials with different indices of refraction which may or may not alsobe thermo-optic in nature. Thermo-optic material as described hereinmeans a material that has at least one optical property (e.g. its n or kvalue) that varies with temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages will beapparent from the following description of particular embodiments of theinvention, as illustrated in the accompanying drawings in which likereference characters refer to the same parts throughout the differentviews. The drawings are not necessarily to scale, emphasis instead beingplaced upon illustrating the principles of various embodiments of theinvention.

FIG. 1 is a block diagram of a laser system;

FIGS. 2 and 3 are plots of wavelength-dependent transmission peaks;

FIGS. 4 and 5 are schematic diagrams of laser systems showing details ofthermo-optical filters;

FIGS. 6 and 7 are schematic diagrams of filter structures;

FIGS. 8-10 are diagrams showing MEMS construction of a filter structure;

FIGS. 11-12 are schematic diagrams of multi-layer thin-film filters;

FIGS. 13(a) and 13(b) are schematic diagrams of filter structures;

FIGS. 14-15 are diagrams depicting a layout of an integrated circuitstructure;

FIG. 16 is a flow diagram of a MEMs-based method of manufacturing afilter structure;

FIGS. 17-26 are diagrams depicting a MEMS workpiece at different stagesof the manufacturing method of FIG. 16;

FIGS. 27(a) and 27(b) are schematic diagrams of a TOT-QCL laser activefilter area.

DETAILED DESCRIPTION

FIG. 1 depicts a tunable laser system 10 as including a quantum cascade(QC) laser element 12 and external cavity components including a lens 4,reflector 14 and a thermo-optic filter 16. Operation is controlled by acontroller 18 which provides drive current to the QC laser 12 and heatercontrol signals to the thermo-optic filter 16. The controller 18typically operates in response to inputs including higher-level controlinput from a user or system, and feedback (FB) from laser operationwhich may be an indication of measured laser output power, wavelength orspatial mode, operating temperature of the laser or external cavitycomponent(s), laser drive current, for example. The controller mayoperate the laser in pulse or continuous wave (CW) operation as known inthe art. The controller may also generate one or more single pulses of adesired width and amplitude when in continuous wave operation in orderto assist in tuning the laser system. The reflector 14 establishes oneend of the laser cavity. In the illustrated embodiment, the reflector 14is partially transmissive, and light transmitted through the reflector14 constitutes the laser output. In another embodiment (not shown) thereflector may be fully reflective and the laser output generateddirectly from a facet of the QC laser element 12. The QC laser element12 includes a QC laser gain medium configured to form an overall lasercavity with the reflector 14 and filter 16, and thus in some embodimentsincludes a separate facet reflector (not shown in FIG. 1) establishingthe other boundary of the cavity. As generally known in the art, the QClaser element 12 can provide optical gain, and therefore support laseroperation, across a relatively wide range of wavelengths (referred to asthe “gain region”). The thermo-optic filter 16 is a thermally tunablewavelength selector that establishes specific wavelength(s) of laseroperation. This selection of operating wavelength(s) can have both acoarse component and a finer component, as described in more detailbelow.

FIGS. 2 and 3 illustrate two separate approaches to design of the lasersystem of FIG. 1. The vertical arrows represent peaks of transmission ofthe thermo-optic filter 16 as a function of wavelength. The periodicaspect of the transmission peaks characterizes certain types of opticalthin-film filters, as described in more detail below. Also shown are“gain curves” 19 representing a gain-versus-wavelength characteristic ofthe QC laser 12. FIG. 2 shows a case in which the gain region of the QClaser 12 overlaps with only one transmission peak, at a wavelength shownas λ₀. This limited overlap can be achieved by suitable design of the QClaser element 12 and the thermo-optic filter 16, and is an example ofcoarse wavelength selection—the laser system will operate only at thewavelength λ₀ of the one overlapping transmission peak. As describedmore below, the exact value of λ₀ can be varied during operation byadjustment of the temperature of the thermo-optic filter, providing forfiner selection of operating wavelength.

FIG. 3 illustrates another approach in which the gain region of the QClaser 12 overlaps with multiple transmission peaks of a single opticalfilter element. In this case, the thermo-optic filter 16 includes twoseparate elements, one exhibiting a pattern of transmission peaks at afirst periodicity and the other exhibiting a pattern of transmissionpeaks at a second, generally different periodicity. These are shown inFIG. 3 by the solid and dashed arrows respectively. In this case,lasing—under continuous wave operation—occurs at only the wavelength(s)at which both elements have a transmission peak, such as at λ₀ as shown.Under pulse mode operation, lasing generally occurs at wavelengths wherethere is sufficient overlap, scaled by the amount of overlap. One orboth of the filter elements are thermally controlled to achieve finerselection of the wavelength of operation.

FIG. 3 illustrates how a pair of filters may be tuned within the QClaser cavity to generate a spectrally varying laser output, which may beused, for example, in gas sensing. The two filters may be designed withslightly different spectral transmissions such that by controlling thetemperature of one or both filters, an operating wavelength of hightransmission through both filters can be achieved. Depending on the typeof filter, this may be referred to as vernier tuning, and the filtersmay be of an etalon type with slightly different free spectral ranges inorder to achieve the desired optical bandpass and total tuning rangecharacteristics.

A laser incorporating the above principles may be particularlyadvantageous for sensing multiple gases. Instead of the continuoustuning of grating tuned lasers, or the mechanical complexity ofmechanically switched filters, these thermo-optic filters can bedesigned to transition from one target wavelength to another targetwavelength with a duty cycle (% of a repetitive time period at thetargeted measurement wavelengths) that exceeds 90% or 99% with a filterthat is designed for rapid thermal response. For a combined filterdesign with three selectable transmission peaks, two gas lines ofinterest and a reference wavelength may all be included with high dutycycle.

Although FIGS. 2 and 3 correspond to use of one and two filter elementsrespectively, more than two filter elements or a combination filters andother spectrally discriminating devices (e.g. diffraction grating) mayalso be used. Overall, the exact configuration will generally depend onthe wavelength characteristics of the QC laser 12, the combined spectralcharacteristics of the filter elements, and the requirements of thelaser output for a particular application.

FIGS. 4 and 5 show example configurations of both types, i.e., usingonly one filter element and using two filter elements. In each case, theQC laser 12 has a high reflectance (HR) back facet coating 20 and anantireflective (AR) front facet coating 22. The HR back facet coating 20and reflector 14 define the boundaries of the laser cavity, and thereflector 14 is partially transmissive in order to output couple thelaser beam signal, which is collimated by action of lens 24. In FIG. 4 asingle thin-film filter element 26 forms the filter 16 of FIG. 1, whilein FIG. 5 it is formed by a pair of thin-film filter elements 30, 32 asdiscussed above in connection with FIG. 3. In each case, the thin-filmfilter elements 26, 30, 32 are preferably tilted slightly away from theoptical axis to direct reflected light (see FIGS. 6 and 7 describedbelow) out of the cavity. Optically transparent wedge-shaped substrates28, 34, 36 may be used to maintain the desired spacing and orientationof the thin-film filter elements 26, 30, 32 and reflector 14. Generally,for a laser facet aperture which is much less than a millimeter wide andan optical beam that is on the order of a few millimeters in diameter,angles of incidence that are even a few degrees off-normal (e.g. <10degrees) are sufficient to achieve this purpose. Angles much greaterthan 40 degrees, for example, may lead to a reduction of filterperformance, in particular, widening of the filter bandpass.

The laser system may be mounted on a thermally conductive substrate andthermo-electric (TE) cooler (neither shown), and both the QC laser 12and filter 16 are electrically connected to the controller 18 providingelectrical control signals to and from the devices as needed. Thesignals to the filter 16 will typically be controlled to achieve adesired temperature (and therefore optical transmission) over time. Thelaser system may be contained within a hermetic package (e.g.“butterfly” or TO style package), and may have one or more transmissivewindows enabling output of the optical signal from the laser package.

While the QC laser 12 is an element of the disclosed embodiment, othertypes of lasers with the ability to support lasing at multiplewavelengths within a gain region of the electromagnetic spectrum (e.g.visible, near IR and far IR spectrum) may instead be used. For any givenapplication there will be a corresponding selection of external cavitydesign and optical materials for the wavelengths of interest.

In one embodiment, the reflected light reflected off a filter (e.g., 26)may be used to provide an output signal from the laser. The filter maybe designed to achieve a reflectance for providing the desired outputsignal power level. This reflected light may be the primary outputsignal from the laser (i.e. no output coupling from the reflector 14 orother optical elements), or it may be a secondary output signal used tomonitor laser operating characteristics (e.g. power or wavelength). Thesecondary signal may be also be used as part of a control loop (e.g.,the feedback FB shown in FIG. 1) for the laser or filter controlsignals. Since reflected light is generated each time the collimatedbeam is incident on the filter, two reflected output signals may begenerated for each filter element. Reflected light may be bounced offanother element in the cavity (e.g. reflector 14) in order to meetcertain mechanical constants of the tunable laser system.

During operation of the system, both incident and reflected collimatedbeams will be at a desired wavelength of operation. The percentage ofthe collimated beam incident on the filter that is reflected may vary asa function of tuning the TOT filter such that, by way of example, thepower of the reflected collimated beam increases, thereby increasing theintra-cavity losses and reducing the power of output light coupledthrough the partially transmissive mirror 14. The TOT filtertransmission function may be designed such that the percentage change inthe power of the reflected collimated beam changes at a greater ratethan the power of the laser output coupled directly out of the cavityas, for example, through partially transmissive reflector 14. Thisincreased rate of change may be advantageous in providing a feedbacksignal for control of the system.

A reflected collimated beam may be used to generate other types offeedback signals. The reflected collimated beam power represents a lossmechanism within the external cavity and is a component of laserefficiency. When the TOT filter transmission at the wavelength ofoperation is reduced, either by, for example, misalignment of thetransmission peaks of two TOT filters or by operation of the TOT-QCLlaser at a wavelength other than at a peak wavelength of the TOT filter,the power contained in the reflected collimated beams (and this thecavity loss) increases. It may be desirable to operate the laser wherethe ratio of the power in the reflected beam to the output power isminimized. Thus, as the electrical power to the QCL is changed or theTOT-QCL wavelength is changed, the ratio of reflected collimated beampower to output power may be used as a feedback signal to the TOT-QCLcontroller.

Referring to FIG. 5, if one filter (e.g., 30) is fixed (i.e., notthermally tunable) while the other one (e.g., 32) is, spectral coveragemay not be continuous. However, if both filters 30, 32 are tunable,continuous spectral coverage can be achieved.

FIG. 6 shows an embodiment in which the reflector 14 is combined ontothe same structure as filter 26, with the mirror reflective surfaceperpendicular to the optical axis to feed the reflecting light back intothe laser gain medium (QC laser 12) and using a wedged substrate 28 forangling the tunable filter 26 with respect to the laser optical axis.FIG. 6 also shows the above-discussed reflected light 27 from one sideof the filter 26.

FIG. 7 shows another embodiment in which thin-film filters 30, 32 arecombined on opposing surfaces of a common thermally conductive andoptically transparent substrate 40 (e.g. silicon or germanium),preferably using a wedge shape or other sloping surfaces to divertreflected light from each filter 30, 32 out of the external cavity. Inanother embodiment, the common thermally conductive substrate 40 neednot be optically transparent if it contains a “donut hole” for passingthe laser light between the filters on the opposing surfaces over thehole. In this manner, thermally and/or electrically conductivematerials—metals such as aluminum or ceramics such as Al₂O₃—may be usedas the substrate. The common substrate may then be attached to a TEcooler to control the temperature of the substrate or provide anathermalized optical platform for the external cavity. This TE coolermay also be attached to the QC laser 12 to simultaneously assist incontrolling the temperature of the laser. More than two filters may alsobe stacked in a single device, combining filters and wedged shapedsubstrates in a “sandwich” of layers.

More specifically, in FIG. 7 the two thin-film filters 30, 32 arecombined structurally on a common mount 42, typically constructed fromone or more highly thermal conductive metal, ceramic or semiconductormaterials. The structure includes an etalon to provide a reference freespectral range (for determining wavelength of operation) or to providepreferential longitudinal modes for improving the stability of athermo-optically tunable QC laser (TOT-QCL) system. The etalon definingmaterial support arm 40 has one surface with suitably opticallyreflective coatings which forms one reflecting surface of the etalon anda second surface that is thermal and mechanical support structure forone of the filters 30, 32. The etalon cavity 44 may be an air gap or amaterial with a different index of refraction than the etalon definingmaterial. The filters 30, 32 are angled with respect to the optical axiswhereas the etalon is orthogonal to the optical axis. If the etalon isan air gap type, then the gap may contain a vacuum or be filled with agas that provides spectral absorption at reference wavelengths usefulfor maintaining the wavelength calibration and stability of a system.The support arms 40 are wedged shaped and may be designed such that thetunable filters 30, 32 are parallel, are at non-perpendicular arbitraryangles with respect to the optical axis, or are at the same positive andnegative angle with respect to the optical axis such that the opticalbeam does not deviate in accordance with Snell's Law. The carrier 42that holds both the support arms may also hold an output coupling mirror14 which is partially transmissive, thereby holding the tunable filters,etalon (optional) and mirror in optical alignment. In another aspect, alens may be mounted to the carrier, thereby providing a single assemblyfor alignment to a laser and also providing a single athermalizedassembly.

In another embodiment, a thermo-optical filter can be deposited on thesurface of a lens such as lens 24, or on a flexible substrate that maybe bonded to a curved surface of the lens.

FIGS. 8-10 show a general architecture of a tunable filter embodiment.In the embodiment as shown, the optical filter element is fabricated ona silicon-on-insulator (SOI) wafer using microelectromechanical (MEMS)and semiconductor processing techniques. FIG. 8 illustrates oneembodiment in which an optically transmissive resistive sheet heater 50for changing the temperature of the optical filter element 52 iscontained in one or more the layers outside the optical filter element52. Also shown are silicon “handles” 54 (e.g. silicon substrate), heaterelectrical contacts 56 and electrical leads (not shown) separated fromthe handles 54 by a buried oxide (BOX) layer 58. FIGS. 9-10 show anaspect of the embodiment where an optically transmissive resistiveheater 60 (e.g. lowly doped crystalline silicon or germanium) iscontained within the thermo-optic cavity layer(s) of the filter. Alsoshown in FIG. 10 are DBR mirrors 62 of the optical filter element 52. Inanother embodiment (not shown), an optically opaque heater (e.g. metalring heater) is fabricated surrounding the filter element. Opticallytransmissive materials that could be used as resistive heaters includebut are not limited to silicon, germanium, and their alloys. In anotherembodiment (not shown) the filter element 52 is comprised of the SOIdevice layer with suitable thickness for forming the thermo-opticcavity, and with deposited DBR mirrors on the upper and lower surfacesof the layer, The optical stack structure may also be designed toinclude the BOX layer.

FIGS. 11 and 12 show two examples of multi-cavity filters constructedusing even quarter wave (QW) thermo-optic cavity layers of a firstmaterial separated by odd QW spacer layers of second material withgreater or lesser index of refraction, and surrounded with DBR mirrors.FIG. 11 illustrates a design with three cavities, referred to as a“32223 triple peak”, while FIG. 12 illustrates a design with twocavities and referred to as a “4224 double peak”. For filter pairs orsingle cavity etalons that operate together to perform tuning, it isdesirable to have filters with different spacings between transmissionpeaks which may be accomplished by varying the thickness of the cavitylayers or spacer layers or both in a particular filter formula. Thecavity design and the number of cavities required for a particularapplication depend on many design parameters, including the number ofwavelengths to resolve, desired laser output characteristics, thematerials used in the cavities and DBR mirrors, the number of DBR mirrorpairs, and the filter optical-mechanical-thermal design requirements.These layers may be deposited on a substrate using industry standarddeposition techniques (e.g. PECVD, IBS, e-beam, etc. depending on thematerials being deposited).

Triple-cavity designs such as that of FIG. 11 can be used to measurethree discrete wavelengths, which can be useful in the detection of NOand NO₂ gas species for example. Another triple cavity design utilizes atwo-mirror pair, triple 4QW cavity, 3QW spacer construction. Examples ofmaterials used for the high and low index layers include, but are notlimited to, amorphous silicon and Ta₂O₅, respectively. Alternatively,both high and low index layers used in mirror pairs may be constructedof materials neither of which are dielectrics. In fact, they may both besemiconductors (e.g. silicon and germanium), as long as there is indexcontrast at the wavelength of interest. Care must be taken in theselection of materials to match the wavelengths of interest since evenextinction coefficients as low as 10⁻³ can lead to substantial filtertransmission loss inside a resonant optical structure.

The thermal time constant associated with heating and cooling of thefilter is determined by the thermal conductance between the filter andits surroundings and the effective thermal mass of the filter. A filterthat is thermally isolated from its surroundings (e.g. seemicrobolometer structures) requires less power to change filtertemperature but also reduces the response time of the filter. Similarly,a thicker filter will typically have higher thermal mass and a reducedresponse time (i.e. longer time constant).

The filter may comprise at least one of a heater element and temperaturesensing element. One or more heaters may be used on a single filterdevice. This would allow for the deliberate creation or compensation oftemperature gradients across the filter separate from the control of theaverage temperature of the filter.

The heater may consist of a patterned metal and the temperature sensingelement (e.g. RTD) may consist of a patterned metal, where thepatterning provides the desired electrical and thermal properties.Patterning may be achieved using techniques such as photolithography orshadow mask. Deposited Pt, Ni, Cr may be used as heater material. Sincethese materials are optically opaque, they would likely be patternedoutside of the beam path.

The heater may also be made by diffusion doping the semiconductormaterial (e.g. Si, Ge). Resistor patterns include but are not limited toring heaters, serpentine heaters, or sheet heaters.

The heater and temperature sensor may be used in a control loop to setthe filter at a desired temperature over time. In one aspect ofoperation, the temperature may be maintained at a specific temperatureover time. In another aspect of operation, the temperature may be rampedup and followed by a cool down or settling period. In anotherembodiment, a TE cooler thermally coupled to the filter may be used tolower the temperature of the filter below ambient temperature or toaccelerate a cooling cycle of the filter.

During operation, a TOT filter may experience a thermal gradient acrossthe active filter element (i.e. that portion of the filter with tunabletransmission and interacting optically with the laser beam). In oneexample, the filter and sheet heater are defined as a wide strip on topof a thin thermally insulating membrane which is thermally attached(i.e. grounded to a heat sink) on all four sides (e.g. FIG. 8 whereinthe BOX layer 58 extends underneath the heater 50). A filter temperaturegradient may broaden the spectral bandpass and increase the averagetransmission loss of the filter since the peak transmission wavelengthwill vary across the plane of the filter and therefor across thecollimated beam incident on the filter. Thermal gradients may arise frommultiple causes, including the mechanical design and thermal isolationstructure of the filter, the amount and physical location of the thermalconductivity paths, absorptive heating of the laser beam in the filter,heating of a temperature sensor, and nonuniformities induced by theheater. In one embodiment, the shape and resistivity of the heater orthe thermal isolation region may be designed to reduce non-uniformitiesinduced by other sources such as laser absorption. For example, a heaterthat surrounds the filter or is embedded throughout the filter on athermally isolated membrane may have regions of higher and lowerresistivity, or higher and lower pattern density, to provide non-uniformheating of the membrane in such a manner as to counteract non-uniformheat loss through thermal isolation structures. Similarly a transparentheater in contact with one surface of the filter may be designed to havenonuniform thickness or physical placement or patterning such as tocounteract heating of the filter by absorption of the laser light. Thecollimated beam may also be non-uniform in nature, as may result from anasymmetrical beam, Gaussian beam shape, or presence of multipletransverse spatial modes. The heater may be shaped to reduce thetemperature gradients resulting from non-uniform absorptive heating thatmay result from such collimated beam.

Additionally, as shown in FIGS. 13(a) and 13(b), the filter transmissionmay be constructed such that it varies spatially across the filtersurface (i.e. perpendicular to the optical axis) in such a manner as tocounteract the thermal nonuniformities and maintain the required filteroptical properties. As an example, thermal non-uniformity which mightconcentrate higher temperatures (and thus longer wavelengths) at thecenter of the filter could be compensated by deliberate film depositionwhere the center of the filter is slightly thinner (shorter wavelengths)than the edges of the filter. FIGS. 13(a) and 13(b) (not to scale) showan embodiment of a spatially varying filter 52 with thermo-optic cavity62 and DBR mirror pairs 62 on the upper and lower surfaces of the cavity62. In FIG. 13(a), the spatial variation is a series of discrete stepsin thickness of the cavity 62. In FIG. 13(b), the spatial variation is acontinuous variation in thickness of the cavity 62. This can beaccomplished using photolithographic photoresist developing (e.g.partial photoresist development to create a spatial variation in etchingrates), etching, deposition and masking techniques as known in the art.The discrete thickness steps of the filter of FIG. 13(a) may act as adiffractive grating that changes the transmitted and reflected light ofthe filter. The step size, spacing and shape (e.g. rectangular,circular, etc.) may be designed to perform as a non-thermally tunableoptical element or grating within the TOT-QCL to disperse light atcertain wavelengths.

FIGS. 14 and 15 show an example TOT filter layout with bonding pads 70,metal connective lines 72, a thermal isolation region 78 comprised ofpatterned thermal isolation legs 80, and thermal isolation platform 82comprised of a heater 74, two four-point measurement resistancetemperature detector (RTD) temperature sensors (not shown), andthermo-optic filter region 76. In this aspect, the layout has symmetryto reduced thermal gradients resulting from the thermal conductivity ofthe leads for the heater, RTD temperature sensors and bonding pads. Inanother embodiment, the thermal isolation region may be nonpatterned anda solid sheet of one or more materials. In another embodiment, thethermal isolation region 78 may be patterned with isolation legs 80 ofdifferent geometries to change the thermal isolation of the thermo opticfilter region 76 and in this manner change the temporal response of thefilter and the power required in the heater for to achieve a givenchange in filter temperature. FIG. 15 shows another embodiment of thethermal isolation platform 82 with the ring heater 74 and a gradientheater 84 (connective leads 72 omitted for clarity). The gradient heater82 may be advantageous for increasing or decreasing thermal gradients inthermo-optic filter region 76 as will be discussed.

In the embodiment as shown the heater 74 is a “ring” type heatersurrounding the filter 76 on the thermal isolation platform 82 as willbe apparent in subsequent figures. Similarly, in this embodiment two RTDtype temperature sensors are placed in proximity to the heater on thethermal isolation platform 82. Fewer or more temperature sensors andother types of sensors may also be used. Similar to the prior heaterembodiments, instead of or in addition to the ring heater, the heatermay comprise one or more layers within the thermo optic filter region76. The heater may also be an optically transparent layer placed insideor outside the thermo optic filter region 76. The temperature sensor maysimilarly be constructed by use of a material with the desiredproperties. For example, silicon of the appropriate doping, thicknessand shape may be used as a temperature sensor and also be a materialforming a layer a filter mirror or quarter wavelength cavity.

FIG. 16 through FIG. 26 describe a fabrication process for a filteraspect where the filter cavity is approximately defined by the devicelayer of an SOI silicon wafer. It is illustrative of the techniques usedto fabricate the filter aspects described herein. In the embodiment asshown, the thermal isolation region comprise the mirror layers of thefilter and in this manner the mirror materials function as the thermalisolation structure, as well as the optical filter structure andmechanical support to the devices on the filter region. Otherembodiments are also possible, including the use of fewer mirror layerswithin the thermal isolation region in order to change the thermalisolation, or the use of one or more cavity layers to achieve a similarresult. In the illustrated embodiment, the electrical leads of theheater and temperature sensor also cross the thermal isolation regionand are patterned to increase thermal resistance while maintainingacceptable electrical conductivity. The patterned leads thus comprisethe thermal isolation region and in one embodiment may exclusivelycomprise the thermal isolation region.

As shown in FIG. 16, the fabrication process includes the followingsteps:

-   -   1. At 80, performing front-side silicon machining. This includes        use of KOH for sloped sidewalls, and stripping and redepositing        LPCVD nitride unless it is smooth and undamaged. FIGS. 17-19        illustrate intermediate stages of processing.    -   2. At 82, front-side heater integration. This includes Ti        metallization and patterning over a roughly 50 um sloped        topology. This is shown in FIG. 20.    -   3. At 84, depositing the front-side mirror, which may be of        SiNx/a-Si or Ta₂O₅/a-Si or

Ge/Si. This step includes etching contact pad holes and possiblydefining support bridges for higher thermal resistance. This is shown inFIG. 21.

-   -   4. At 86, back-side silicon machining. This includes protecting        the front side using a hard-cured polyimide, then a sequence of        patterning nitride hard mask, KOH, and BOE. This is illustrated        in FIGS. 22-23.    -   5. At 88, depositing the back-side mirror, which again may be of        SiNx/a-Si or Ta₂O₅/a-Si or Ge/Si. This is a blanket coat—no        patterning—as shown in FIG. 24.    -   6. At 90, completing the process, which will typically include        dicing and other finishing steps. This is shown in FIGS. 25-26.        The filters can be integrated as shown in FIG. 26. In this        embodiment, the filter die are bonded together, either at the        die or wafer level using anyone of a number of techniques known        to those conversant in MEMS technology.

As described previously, the thermal isolation region may be patternedto achieve a desired thermal isolation of the filter island. The shapeof the patterning may be determined by considerations of material,process and layout considerations. The shape may also be determined inorder to achieve a certain thermal profile or gradient across the filterisland. In one embodiment, one or more of the shape and location of theheater, laser beam, filter thermal isolation platform, electrical leads,bonding pads, and thermal isolation region may be used to reduce thermalgradients across the filter. In one embodiment, as an example, theelectrical leads and bonding pads are located and patterned such as tohave symmetry and thus reduced thermal gradients in the filter island,thermal isolation region and the region surrounding or attached to thethermal isolation region on the side opposing the filter island.

Certain Specifics and Alternatives

A TOT-QCL laser wherein the filter element optical area is larger thanthe collimated beam of the laser. In one embodiment, the filter diameteris at least 22% larger than the 1/e² diameter of the beam (diameter inthis context refers generally to the two dimensional spatial extent ofthe beam orthogonal to the direction of beam propagation and is notmeant to strictly imply a circular or symmetrical beam cross section).This may be advantageous in improving the thermal uniformity of thefilter.

A TOT-QCL laser as shown in FIG. 27 wherein the active filter regionarea 110 is less than the area of the beam of the laser 112 and withfilter regions within the optical path of the laser beam (excluding theactive filter region) being non-tunable filter regions 114 of differentoptical transmission. These non-tunable filter regions 114 may have hightransmission to ensure good average optical transmission in band. Thenon-tunable filter regions 114 may surround the filter as shown in FIG.27a or be interspersed in the filter in a checker board or other patternas can be achieving using photolithographic techniques as shown in FIG.27b . In one embodiment, the active filter area 110 is at least 22%smaller than the 1/e2 diameter (or area so inscribed) of the beam. Thisaspect may be advantageous in reducing the thermal mass of the filter inimproving the uniformity or creating a desired level of wavelengthselection and cavity loss. In another embodiment, the non-tunable filterregions may be tunable but with different tunable transmissionproperties than the active region.

A TOT filter in which one or more of the materials comprising the cavityor DBR mirrors is thermally conductive to provide improved thermaluniformity and thus wavelength uniformity across the filter.

A TOT filter in which a thermally conductive and optically transparentmaterial is added to the filter structure to provide improved thermaluniformity across the filter. In another aspect, this material may alsobe electrically conductive. In another aspect, this material may bepartially reflective. Silicon or germanium may be used.

A TOT-QCL laser wherein the filter has been designed for increasedtransmission or improved finesse by using curved surfaces for when theincident light is only partially collimated or uncollimated.

A TOT-QCL laser package or integrated inter-filter spacing 200 of FIG.26 that has a backfill gas that also is used for one or more of thefollowing: (i) to act as a filter for the purpose of generating areference or calibration signal for the laser; (ii) to act as filter indetermining, at least in part, the output wavelength of the laser, (iii)to alter the time response of the filter during heating or cooling. Onecan then trade-off input power for tuning speed.

Use of the buried oxide in an SOI wafer as an etch stop for backsideetching, the buried oxide also being a part of the opto-mechanicaldesign of an optical filter.

A TOT filter mechanically attached to the facet of a QC laser, and whichmay also be optically coupled with a material with an index ofrefraction equal to or greater than 1. The optically coupled materialmay be designed with a material with a low thermal conductivity relativeto silicon or with materials used in the optical cavity of the QCL. Sucha filter may be curved or vary in thickness to accommodate the divergentoptical output of the QC laser.

A TOT filter with a spatial thickness gradient nominally perpendicularto the optical axis that compensates for one or more of the following:(i) a thermal gradient in the filter; (ii) a diverging or convergingbeam incident on the filter.

A TOT filter with a spatial thickness gradient nominally perpendicularto the optical axis that is fabricated using photolithographictechniques. The thickness gradient may be uniformly positive or negative(i.e. the change in thickness from the center of the filter to the edgeis constant) or the gradient may change in discrete positive or negativesteps (e.g. through the use of discrete photolithographic etching stepsas commonly used in MEMS processing.

A TOT-QCL with a thermally controlled element in the laser cavity thatvaries the optical cavity path length by thermally changing the index ofrefraction of the element. In one embodiment, a single layer ofthermo-optic material is AR coated on both sides, and has an integratedheater which is used to change the temperature and thus the optical pathlength of the element.

Thermo-optic filters in a TOT-QCL that pulse the laser on and off, orsubstantially modulate laser output, by changing the temperature of thefilter or filters and therefor their transmission, thereby enablingpulse operation or time variant operation of the laser without varyingthe electrical power supplied to the laser. This aspect may also be usedas an optical modulator for AC coupled detectors, such as pyroelectricinfrared detectors.

Thermo-optic filters in a TOT-QCL wherein both the temperature of thefilter and the power to the laser are synchronously controlled toachieve a desired spectral output of the laser. For example, for a laseroperated in pulse mode, by pulsing the laser electrical power, andcontrolling the temperature of the filters, the laser output may bemodulated such that lasing is achieved for a period of time less thanthat which would be achieved in the absence of the thermal filters. Inthe same manner, the spectral linewidth or number of laser modes duringthe pulse may also be controlled.

A TOT-QCL wherein the electrical power to the QCL or the lasertemperature is changed as a function of the temperature of the TOTfilter or filters such that a desired laser output characteristic isachieved. Such an output characteristic may include one or more of thelaser power level, laser longitudinal or transverse mode, number ofsimultaneous laser modes, and laser wavelength.

A TOT-QCL wherein the electrical power to the filter or the filtertemperature is changed as a function of the laser temperature or laserelectrical power such that a desired laser output characteristic isachieved. Such an output characteristic may include one or more of thelaser power level, laser longitudinal or transverse mode, number ofsimultaneous laser modes, and laser wavelength.

A TOT-QCL wherein the gain medium of the QCL (1) achieves lasing in aspectral region where one or more substances of interest have spectralfeatures of interest, and (2) the laser gain medium has a gain profilethat is narrower than the spacing between transmission peaks of a TOTfilter with multiple transmission peaks (e.g. the free spectral rangefor an etalon type filter). When one of the TOT filter peaks ispositioned within the laser gain window, only those laser modes underthat single TOT filter peak will lase. In this manner fewer TOT filtersmay be used in a TOT-QCL. The QCL may be designed specifically to matchthe TOT filter characteristics and may also be designed to operate inmore than one spectral region of interest, that is have multiple gainregions, each with a width less than the free spectral range andseparated by a region of low gain that does not support lasing.

A TOT-QCL where the absorption of laser light changes the temperature ofa TOT filter thereby changing the wavelength of peak transmission of thefilter and the preferred lasing wavelength. For a laser operatingcontinuous wave, the laser may remain operating at a first wavelengthwhen the TOT filter transmission peak moves to a second wavelength,thereby resulting in increased cavity losses and a reduced laser output.Similarly, for a laser operating continuous wave at wavelength not atthe peak of the TOT filter, the laser may remain operating at a firstwavelength when the TOT filter transmission peak moves to a secondwavelength, thereby resulting in reduced cavity losses and an increasedlaser output.

When combined with a second TOT filter, the change in transmissionwavelength in the first TOT filter due to laser absorption may increaseor decrease the laser output power depending on the relative alignmentof the peak transmission of the two TOT filters. By way of illustration,if the transmission peak wavelength of one TOT filter is aligned on theslope of the transmission peak of a second TOT filter, small changes intransmission wavelength of one filter with respect to the second filtermay result in relatively large changes in insertion loss and thus laserpower. This aspect may be useful in spectroscopic applications toprovide additional gain in modulation of laser power in either CW orpulse operation. In another aspect of the design, the laser inputelectrical signals may be modulated. A TOT-QCL where the absorption oflaser light changes the temperature of a TOT filter in a spatiallynon-uniform manner thereby broadening or narrowing the wavelengthlinewidth (e.g. number of longitudinal modes) of the laser output. Bychanging the current (i.e. electrical power) to the QCL, the laser powerwill change and thus the amount of absorption induced line broadeningwill change. This aspect may be useful in spectroscopic applications toprovide varying probe linewidths in either CW or pulse operation whilemaintaining a constant laser center wavelength. In another aspect of thedesign, the TOT filter temperature may be changed in response to thechange in laser power to maintain a preferred transmission wavelength.

A TOT-QCL where a TOT filter is used to maintain a constant wavelengthof operation for varying QCL temperatures, varying QCL power levels orvarying ambient or other environmental conditions.

A TOT-QCL where a TOT filter and a second optical filter are used tomaintain a constant wavelength of operation for varying QCLtemperatures, varying QCL power levels, varying ambient temperature orother environmental conditions. A modulated QCL power may be desired toprovide an optical signal to an AC coupled infrared detector, such as apyroelectric or acoustic detector, or it may be used to improve systemsensitivity by changing the detection bandwidth, such as performed witha lock-in amplifier or spectral signal processing.

A TOT-QCL further comprising two filters: a TOT filter and a filter witha substantially lower thermo-optic coefficient thereby eliminated theneed to control the temperature of the second filter. The second largelyathermalized filter may be used to restrict the number of usable TOTfilter transmission peaks falling within the gain curve of the QCL. Byway of example, an air gap etalon may be used as an athermalized etalon.

A TOT-QCL further comprising a filter and a grating. An issue with agrating tuned external cavity QCL using mechanical motion for tuning isthe difficulty of achieving wavelength stability and repeatability. Agrating and TOT filter wherein a grating provides a coarse tuningcapability over a broad spectral range and the TOT filter provides finetuning at each mechanical position of the grating provides improvedwavelength stability and repeatability. The tuning range of the TOTfilter would be designed to exceed the coarse tuning step size and theamount of wavelength uncertainty or repeatability error in a gratingonly design. For example, a grating tuned QCL may have 0.2 cm-1uncertainty in wavelength over time and TOT filter may have a tuningrange of 0.4 cm-1. In one embodiment the filter and grating may becombined on a common substrate

In another aspect of a TOT-QCL, a second heater may be added to thefirst heater of the TOT filter to induce a thermal gradient across thefilter and thereby change the effective filter finesse and laserlinewidth. The power to the second heater may be constant or timevariant thereby either setting a finesse and linewidth or inducing atime varying finesse and line width. In this manner the number of laserlongitudinal modes, reflected collimated beam power or laser outputpower may be changed dynamically, with or without a change inwavelength. The heater may take the form a strip along one side thefilter that results in an approximate one dimensional gradient from oneside of the filter to the other. Alternatively, the heater may take theform of a “point” heater in the center of the filter providing a radialthermal gradient from the center to the periphery of the filter. Thefirst heater and second heater may be controlled together to maintain aconstant average transmission wavelength as the finesse is modulated.

In another aspect of a TOT-QCL, membrane filters/etalons can deform,allowing them to act like a lens. This can be done deliberately toimprove or degrade the external cavity, making the laser turn on or off,as well as shift the center wavelength. Mechanical deformation can beachieved either by thermally induced mismatch of thermal expansion, orby pressure differences across the membrane.

In another aspect of the TOT-QCL, a single filter element with multiplespectral transmission peaks (“multi-peak”) within the laser gain regionmay be used within the cavity to select more than one simultaneouslasing mode, where these lasing modes are not adjacent or in successiveorder of wavelength. For a laser gain medium that could supportsimultaneous lasing at lambda1, lambda2, and lambda3 wavelengths, it maybe useful for a single laser to support simultaneous lasing at lambda1and lambda3 but not at lambda2, and this could be achieved byincorporating a TOT filter element with transmission peaks at lambda1and lambda3 but not at lambda2. Wavelength discrimination may beachieved at a laser output power detector by a dichroic filter/mirror orby use of discrete filters.

In another aspect of the TOT-QCL, a TOT filter with multiple spectraltransmission peaks (“multi-peak”) within the laser gain region may beused within the laser cavity to select more than one non-simultaneouslasing wavelength, wherein the TOT filter temperature remains constant.One or more changes in laser operating conditions may be used to achievethe non-simultaneous lasing, including operating as a pulse laser (i.e.the laser switches on and off), changing pulse duty cycle, changingpulse duration, changing laser power, or changing laser temperature. Byway of example, consider a multi-peak filter (or combinations offilters) with bandpasses at wavelength 1 and wavelength 2. If the lasertemperature is changed when operating at wavelength 1, then a differentwavelength of operation at wavelength 3 may be induced. However, withthe TOT filter inserted into the laser cavity, wavelength 3 is lesspreferential for laser operation than wavelength 2 and the laser wouldoperate at wavelength 2. In this manner certain wavelengths of operationmay be preferentially selected or excluded as, for example, may bedesired to select certain gas absorption lines in combination withcertain reference wavelengths.

A multi-peak TOT filter can be achieved by vernier tuning of two TOTfilters such that two wavelengths have equivalent transmissioncharacteristics. It then becomes possible to rapidly switch between thetwo wavelengths by either small changes in TOT filter tuning or byinducing some other change in operating conditions, such as changing thelaser operating temperature or drive current thereby changing the QCLgain curve characteristics. When operating in CW mode, it may beadvantageous to pulse the laser current to assist in snapping to the newwavelength by breaking the preference of a CW laser to maintain itscurrent operating wavelength despite the creation of more favorableoperating conditions at another wavelength. Multi-peak filters encompassthe possibility of more than two peaks using different types of filtersor combinations of filters as previously discussed.

Those skilled in the art will recognize that a non-tunable multi-passfilter can achieve these results albeit without the ability todynamically adjust for new wavelengths or operating conditions. However,at tunable TOT filter may be preferred. For example, due to changes inenvironmental conditions (e.g. external cavity temperature) or lasergain profile (as may occur with aging or burn in), a TOT filter canadjust its passband characteristics to compensate for the changes toensure the desired operating wavelengths or power are maintained overtime and environment.

In another aspect of the TOT-QCL, a TOT filter with multiple spectraltransmission peaks (“multi-peak”) within the laser gain region whereinthe filter transmission peaks may be designed to match the laser gainwavelength curve or vice versa) in order to achieve lasing at more thanone wavelength, either simultaneously or as a result of changes in theTOT-QCL operating conditions (e.g. laser power, pulse duration, powerlevel or temperature).

In many industrial applications, laser stability is an importantcriteria for achieving reliable operation, signal to noise improvementsthrough background scans or averaging, and low maintenance costs. Thustechniques for improving the stability of the laser system areimportant. One such technique is to reduce the sensitivity of the systemto environmental or system perturbations through the design of one ormore elements of the system. Lens athermalization is one example of thistechnique in optical design.

Consider a TOT-QCL in which certain longitudinal cavity modes areenhanced such that small changes in certain operational conditions donot result in changes in laser wavelength (an EM-QCL). For example, alongitudinal mode may be created by the reflections off a facet of theQC laser (e.g. an optical resonance or cavity modes are created withinthe QCL by facet reflections off the two ends of the QCL, or between theQCL facet and other partially or fully reflective surfaces within theexternal cavity). While a good anti-reflection coating on the cavityfacet (surface) facing the external cavity may be desired to reduce thestrength of this mode, it can also be strengthened by providing greaterreflection off the facet surface. The laser will thus have an increasingpreference to only lase at those wavelengths supported by the QCL cavitymodes. Thus, as the TOT filter changes temperature (or, for example, theeffective length of the optical cavity changes as a function of changesin the temperature of the cavity structure) the laser will stay at onewavelength until the temperature change is sufficient to create apreferred lasing condition at another QCL cavity mode. By way ofexample, for a typical QCL operating at 4.5 um, the QCL mode spacing maybe 1 nm, and the change in TOT filter temperature to transition fromlongitudinal mode to longitudinal mode may exceed 5 degrees Celsius.Thus, the temperature stability requirement for the TOT filter is nowreduced. Importantly, also reduced is the absolute temperature accuracyover the entire temperature range of the TOT filter. For example, if theTOT filter operates between 400° C. and 500° C., it requires a 5° C.temperature change at 500° C. to jump cavity modes, the system onlyneeds to hold the long term temperature accuracy to 2.5° C. However, thetolerances for short term stability and long term repeatability are notindependent and must be allocated and traded off in the system design.

A facet of the QCL may be uncoated in order to enhance the facetreflectivity. The coating may be deposited on the facet with areflectivity of greater than 1% to enhance a longitudinal cavity mode. Atrade-off exists between the strength of the mode selection (byincreasing the facet reflectivity) and the increase in laser thresholdthat occurs as the TOT filter is tuned away from the mode wavelength(which can result in a change in laser power as the TOT filter tunes).

For some applications, a longitudinal spacing of more or less than 1 nmmay be desirable. Other surfaces within the external cavity may then beused to obtain the desired spacing. Alternatively, a mode creatingoptical element, such as an etalon or other filter structure may beintroduced into the cavity with the right mode spacing and surfacereflectivity to achieve the desired wavelength and mode strength. Thisoptical element may be athermalized or mounted to a TE cooler to achievethe desired temperature stability and repeatability. The mode spacingmay be further adjusted by angling the element with respect to theoptical axis, and either fixed in place or made adjustable dynamicallythrough mechanical means (e.g. a piezoelectric driver).

A laser may have more than one mode enhancing structure. For example,there are longitudinal modes created by the laser chip as justdescribed, by the EC (external cavity) reflective (or output coupling)mirror and another at least partially reflective surface, and by otheroptical surfaces within the EC. A TOT-QCL may be designed to havemultiple mode creating structures at different wavelengths (i.e.spacings) and at different strengths (e.g. the etalon cavity 44 of FIG.7). For example, a set of weak cavity modes may be created by surfaceswith relatively good anti-reflection coatings and a short wavelengthspacing between modes (i.e. a thick low Q etalon) while a second set ofstrong cavity modes may be created at a wavelength spacing greater thanthe first set (i.e. a QC chip with poor AR facet coatings). The firstset of cavity modes may be spaced at a wavelength to achieve a desiredspectral resolution of the laser. The second set of cavity modes may bea spacing that facilitates calibration of the laser over greaterwavelengths, longer periods of time, over changes in environmentalconditions or for other reasons. Different combinations of mode spacingsand mode strengths can thus be envisioned, each with particularadvantages with respect to maintaining a desired wavelength or powerrepeatability, stability or spacing of laser wavelengths, or aparticular level of power modulation over laser operating conditions.

In the EM-QCL as described, the laser may be operated in pulse or CWoperation. If operated in CW, the laser may “lock” onto a givenwavelength such that as the TOT filter is used to tune the laser, thelaser “sticks” on a given frequency (and longitudinal mode), thusforcing the TOT filter to tune even beyond the point where the peak ofthe TOT filter transmission aligns with the next longitudinal, mode toachieve a change in lasing wavelength at the adjacent longitudinal mode.This has the further disadvantage that as the TOT filter tunes furtherfrom the optimal wavelength for the current wavelength of operation, thethreshold power increases and the output power from the laser isreduced. The laser operating in CW mode may be pulsed in order tomitigate or eliminate this effect by allowing the laser to lase, whenpulsed back on, at the wavelength where the TOT filter and longitudinalcavity modes are most closely aligned.

Thus, a method of tuning the TOT-QCL laser in CW operation is asfollows:

1. Establish a first wavelength of operation of the laser where alongitudinal cavity mode of the laser and the TOT filter bandpass arealigned (note: the TOT filter may consist of one or more optical filtersand the longitudinal cavity mode may be an effective mode created by acombination of mode creating surfaces within the TOT-QCL).

2. Adjust the TOT filter to the wavelength location of a secondlongitudinal mode for the laser, the laser continuing to lase at thefirst wavelength of operation

3. Reduce the laser electrical power in single pulse such that the laserlases at the wavelength corresponding to the second longitudinal mode ofthe laser

In another aspect, the laser is continually pulsed at a very long dutycycle (i.e. 99.9% full power, 1% reduced power) and short pulse time(i.e. 1 usec) relative to the time that the laser is being used tocollect data. This replaces step 3 as just described.

In another aspect, the alignment of the cavity mode to the TOT filtertransmission peak is determined by a method of varying the TOT filterwavelength to maximize the laser power.

In another aspect, a control loop is used to maintain the maximum outputpower at a given cavity mode wavelength wherein the control is achievedby 1) setting a desired wavelength of operation; 2) monitoring the laseroutput power with a detector, and 3) changing the temperature of the TOTfilter to achieve a desired power, or power and wavelength, outcome.

In another aspect, the lasing wavelength of the TOT-QCL is changed bychanging the temperature or other operating condition of the cavity modedetermining device. By way of example, the cavity mode determiningdevice may be the QC laser wherein the cavity modes of the laser are thechip modes of the QC laser, determined at least in part by the lengthand temperature of the QC laser. The temperature of the TOT filter maybe changed at the same time as the change in cavity mode wavelength inorder to keep the wavelengths of the cavity mode and TOT filter aligned.A control loop may be used to maintain the alignment wherein either theTOT filter or cavity mode determining device is adjusted to maintainmaximum power.

In another aspect, the control loop will operate to maintain constantpower rather than maximum power. By this means the laser power may beheld constant at each wavelength where the laser is used to generatemeasurement data.

In another aspect, the wavelength of the cavity mode is changed bychanging the current input to the QC laser. A control loop may be usedto maintain constant laser output power through control of the TOTfilter as the laser current is changed. By way of example, adjusting thewavelength of the transmission peaks of two TOT filters relative to eachother, the center wavelength and the FWHM bandpass of the combinedfilter transmission can be controlled. In another aspect, a control loopmay be used to maintain constant laser output power through control ofthe laser temperature as the QCL current is changed. In another aspect,a control loop may be used to control the laser input current as thetemperature of the QCL laser is changed.

In another embodiment, certain optical absorption features within theTOT filter (cavity or DBR materials) may be used to provide a wavelengthreference. For example, a TOT filter comprised of amorphous silicon isknown to have an absorption feature at around 5 um. The TOT filter mayalso be designed with materials with specific absorption features. Forexample, a TOT filter with silicon as its optical cavity may have thesilicon doped with an impurity that provides one or more absorptionfeatures that may be used as an optical reference for calibrating theTOT-QCL as it sweeps across its tunable wavelengths (i.e. the filterwill have reduced transmission at the absorption wavelength(s),resulting in a modulation in laser output power relative to thenon-absorptive wavelengths). Alternatively, a coating may be depositedon the filter to achieve absorption features.

The mode creating optical element may also be used as an opticalreference within the system.

The filters can be highly integrated as discussed previously for FIG.26. In this embodiment, the filter die are bonded together, either atthe die or wafer level using any one of a number of techniques known tothose conversant in MEMS technology. Also using MEMS process techniques,it is possible to have the gap or cavity between the filters be sealedor relieved with holes to allow gas to readily enter the space betweenthe filters. It may be advantageous to have the gap sealed and evacuated(as is readily possible using wafer bonding techniques) or back filledwith a known gas. The gas may have a thermal conductivity that is higheror lower than that of nitrogen. The gas may also have spectralabsorption at reference wavelengths useful for maintaining thewavelength calibration and stability of a system. Stacking of more thantwo filters is also possible, provided spaces or etched cavities areused to avoid thermal contact between thermally isolated filter elementsand semiconductor techniques for enabling electrical contact to buriedleads are used. The bonded filters as a single assembly may then bethermally attached to a thermally conductive substrate or mount

The gain curve of the QCL may also provide a reference wavelength. TheQCL gain curve contains at least one peak (FIG. 2) or other spectralfeature, which results in a peak or spectral features in the poweroutput of the TOT-QCL. The gain curve of the QCL may be constructed toprovide features within the gain curve to act as wavelength references.As the TOT-QCL is tuned across the gain curve, the resulting powerspectral density output of the laser may be analyzed to determine thelocation of a known QCL spectral feature with respect to the temperatureof the TOT QCL filter. In this manner, the calibration of TOT filtertemperature versus laser wavelength can be recalibrated over time toaccount for drifts or instabilities in the TOT-QCL system. Additionalspectral reference points can be used to increase the accuracy of thereference calibration.

It is well known that some lasers exhibit transverse as well aslongitudinal modes. A QCL may have a single transverse mode (e.g. TEM00)or it may exhibit multiple transverse modes. In many applications, asingle TEM00 mode is preferred. However, a given QCL, due to design orfabrication, may exhibit non-TEM00 behavior. A TOT filter in an externalcavity may be used to provide feedback into the QCL laser cavity topreferentially select a preferred transverse mode of operation.Preferential feedback may be achieved by providing higher intra cavitygain for the preferred mode of operation. The TOT filter may be designedwith a spatial gradient orthogonal to the optical path to increase theloss of certain transverse laser modes. In another aspect, the filtermay be surrounded by a material that absorbs or is non-transmissive atthe operating wavelengths of the QCL such that part of the collimatedbeam is blocked when passing through the filter.

While many of the embodiments described here are useful for use asintra-cavity elements within an external cavity laser, they may also beuseful as tunable wavelength selection filters for lasers or otheroptical sources with spectral outputs at multiple wavelengths, forexample a Fabry Perot QCL. The tunable filter may be used to select oneor more spectral wavelengths of interest for the optical source.

A TOT filter may also be used in conjunction with a QCL to create alaser source emitting simultaneously at multiple wavelengths aspreviously described. By way of example, as previously disclosed, asingle TOT may have multiple passbands spaced at the free spectral rangeof the TOT filter. When used as a tunable element in an EC-QCL, thelaser may be operated so that the laser creates an optical output atmore than one wavelength where the TOT filter passbands and the abovethreshold laser gain curve intersect. The TOT filter may then be tunedover the free spectral range to achieve full spectral coverage, albeitwith multiple laser output wavelengths at each operating point of theTOT filter.

Such a simultaneous wavelength laser source may be used as a highbrightness broadband source as in the spectrometer of Pflugl, et al(US2011/0058176 A1). However, whereas Pflugl describes static broadbandsingle sources (e.g. FP lasers), or multiple narrowly tunable broad bandsources acting as a single source, we describe a single lasersimultaneously generating multiple narrow spectral emissions that arethen tunable over time to generate the desired broadband spectralwavelength coverage.

When combined with a wavelength dispersive element such as aninterferometer, this TOT-QCL simultaneous wavelength source can achievewith a single laser much broader spectral coverage (using scanning andtunability) or the same coverage as an array QC lasers each operating ata different wavelength (possibly without scanning and with narrow FSRs).In comparison to a mode of operation with two TOT filters to achieve asingle wavelength of operation, the simultaneous source andinterferometer eliminates the need for a second TOT filter whileproviding previously unobtainable (with broadband incoherent sources)very narrow high power spectral lines at spectral wavelengths measuredsimultaneously by the interferometer.

When used with an interferometer, the simultaneous source may be tunedat a rate that is slower then, faster then or in phase with the scanningof the interferometer. If scanned slower than the interferometer, than atypical mode of operation would be to collect an interferometer scan,tune the source to a new operating point thereby outputting a new set ofsimultaneous wavelengths, followed by a second interferometer scan.

Aspects of Novelty

Distinct groupings of aspects of novelty are outlined below withrespective numberings 1s, 100s, 200s, 300s, 400s, 500s and 600s.

1. A tunable laser with a laser output, comprising:

a solid state laser medium comprising an optical gain region andoperative to generate coherent radiation through a facet in response toan electrical signal;

a lens operative to collect the coherent radiation and generate acollimated light beam;

a reflective surface for reflecting the collimated beam back to the lensand the laser medium thereby creating an external laser cavity;

an optical filter comprising two surfaces with a thermally tunableoptical transmission band within the optical gain region of the lasermedium, the optical filter positioned within the external laser cavitybetween the reflective surface and lens, and operative to

(1) transmit a collimated beam at a desired wavelength of operation, and

(2) specularly reflect a collimated beam from each surface, thecollimating beam incident on the optical filter such that the reflectedcollimated beams propagate at an angle with respect to the incidentcollimated beam

2. The tunable laser of 1 further comprising a second optical filterwith a thermally tunable transmission band within the operative range ofthe laser medium and positioned within the external laser cavity betweenthe reflective surface and lens, and operative to

(1) transmit the collimated beam at a desired wavelength of operation,and

(2) specularly reflect a collimated beam from each surface, thecollimating beam incident on the optical filter such that the reflectedcollimated beams propagate at an angle with respect to the incidentcollimated beam

(3) transmit radiation in the same band as the first optical filter at afirst wavelength and block transmission at a second wavelength

3. The tunable laser of 2, wherein the first and second optical filtersare etalons, each filter with a different free spectral range andconfigured to operate as tunable vernier filters to generate a tunablelaser4. The tunable laser of 1, wherein the reflective surface is partiallytransmitting to generate the laser output5. The tunable laser of 1, wherein a specularly reflected collimatedbeam is the laser output6. The tunable laser of 1, wherein the power of specularly reflectedcollimated beam varies as a function of thermally tuning the opticalfilter7. The tunable laser of 6, wherein specularly reflected collimated beamis used as a reference signal8. The tunable laser of 1, wherein the tunable filter and reflectivesurface are combined on an optically transparent common substrate9. The tunable laser of 1, wherein the tunable filter and reflectivesurface are combined on a common substrate, the common substratecomprising an aperture for transmitting the collimated beam10. The tunable laser of 1, wherein the power of the specularlyreflected collimated beam has an inverse relationship to the power ofthe laser output11. The tunable laser of 1, wherein the laser gain medium, lens, opticalfilter and reflective surface are mounted to a temperature stabilizedthermally conductive substrate12. The tunable laser of 1, wherein the tunable filter and lens arecombined to form a single assembly13. The tunable laser of 1, wherein the spatial diameter of a thermallytunable transmissive region of the optical filter at the desiredwavelength of operation of the tunable optical filter is at least 22%smaller than the 1/e2 diameter of the collimated beam14. The tunable laser of 13, wherein the tunable transmissive region isfurther comprising a region simultaneously transmissive at allwavelengths of operation of the tunable laser15. The tunable laser of 1, wherein the optical filter is an etalon, thefree spectral range of the etalon exceeding the wavelength width of theoptical gain region of the solid state laser medium16. The tunable laser of 1, wherein the optical filter is an etalon andthe optical gain region of the solid state laser medium having awavelength region of lower gain interspersed between regions of highergain, the region of lower gain corresponding to an optical passband ofthe etalon.17. The tunable laser of 1, wherein the wavelength of the thermallytunable transmission band changes as a result of absorption of thecollimated beam18. The tunable laser of 17 further comprising control circuitry, thecontrol circuitry operative to change the temperature of the opticalfilter in response to the absorption of the collimated beam19. The tunable laser of 1 wherein the optical filter is an etalon, thetunable laser further comprising an athermalized etalon, the freespectral range of the optical filter etalon being less than the freespectral range of the athermalized etalon20. The tunable laser of 1 further comprising an optical grating formechanically tuning the laser output21. The tunable laser of 1 further comprising control electronics andlaser power detector operative to maintain a constant laser output powerby changing the temperature of the optical filter22. The tunable laser of 1 further comprising control electronics andlaser power detector operative to maintain a constant laser output powerby changing power of the specularly reflected collimated beam100. A thermally tunable optical filter comprising

a thermally conductive substrate;

a filter region comprising a thermo-optic material and two distributedBragg reflectors, the filter region having a thickness in the directionof optical propagation less than the thickness of the thermallyconductive substrate;

a thermal isolation region connecting the substrate and filter region,the thermal isolation region having a temperature gradient;

a patterned thin film heater for changing the temperature of the filterregion in response to electrical signals, the heater being connectedthermally to the filter region and isolated from the substrate by thethermal isolation region;

a patterned temperature sensor thermally connected to the filter region;

an optical transmissive wavelength within the filter region that variesin response to electrical signals provided to the heater; and

control circuitry operative to control the heater electrical signals

101. The thermally tunable filter of 100, wherein the thin film heateris optically transmissive and substantially covers the filter region102. The thermally tunable filter of 100, wherein the shape and locationof the thin film heater reduces thermal nonuniformities in the filterregion103. The thermally tunable filter of 100, wherein the opticaltransmission varies spatially in a direction orthogonal to preferreddirection of optical propagation to reduce spatial variation in opticalbandpass104. The thermally tunable filter of 100, wherein the distributed Braggreflectors comprise a thermally conductive layer105. The thermally tunable filter of 100 further comprising a thermallyconductive, optically transparent and electrically conductive layer106. The thermally tunable filter of 100, wherein the filter region is acurved surface107. The thermally tunable filter of 100, wherein the substratecomprises a buried oxide layer108. The thermally tunable filter of 100, wherein the patterned thinfilm heater is patterned to generate a spatially varying opticalbandpass in the optical transmission region109. The thermally tunable filter of 100, wherein the patterned thinfilm heater is patterned to reduce a spatially varying optical bandpassin the optical transmission region110 The thermally tunable filter of 102 further comprised of a secondindependently controlled heater200. An external cavity semiconductor laser comprising

a solid state laser medium operative to generate coherent radiationthrough a first facet in response to an electrical signal;

a lens operative to collect the coherent radiation and generate acollimated light beam;

a reflective surface for reflecting the collimated beam back to the lensand the laser medium thereby creating an external laser cavity;

a thermo-optic thin film filter with a spatial thickness gradientperpendicular to the direction of optical propagation of the collimatedlight beam

201. The external cavity semiconductor laser of 200, wherein thethickness gradient is composed of discrete changes in optical thickness202. The external cavity semiconductor laser of 200, further comprisingthermo-optic material for changing the optical path length of theexternal cavity as a function of the temperature of the thermo-opticmaterial300. An external cavity tunable laser with a laser output comprising

a solid state laser medium operative to generate coherent radiationthrough a first facet in response to a laser electrical signal;

a lens operative to collect the coherent radiation and generate acollimated light beam;

a reflective surface for reflecting the collimated beam back to the lensand the laser medium;

two thermo-optic thin film filters, each with a heater, and togetheroperative as an optical vernier wavelength tuner;

control circuitry operative to generate heater signals for controllingthe temperature of the thermo-optic thin film filters and modulate thevernier optical passband and the laser output power

301. The external cavity tunable laser with a laser output of 300,wherein the modulation of the vernier optical passband turns on and offthe laser output302. The external cavity tunable laser with a laser output of 300,wherein the modulation of the laser output power is substantiallysinusoidal303. The external cavity tunable laser with a laser output of 300,wherein the control circuitry synchronously modulates the vernieroptical passband and the laser electrical signal304. The external cavity tunable laser with a laser output of 300,wherein the laser output is a pulse signal and the synchronousmodulation varies the time duration of the signal pulse305. The external cavity tunable laser with a laser output of 300,wherein the laser temperature is controlled and the control circuitrysynchronously modulates the vernier optical passband and the lasertemperature400. An external cavity tunable laser with a laser output comprising

a solid state laser medium operative to generate coherent radiationthrough a first facet in response to a laser electrical signal;

a lens operative to collect the coherent radiation and generate acollimated light beam;

a reflective surface for reflecting the collimated beam back to the lensand the laser medium;

two thermo-optic thin film filters, each with a heater, and togetheroperative as an optical vernier wavelength tuner;

control circuitry operative to generate heater signals for controllingthe temperature of the thermo-optic thin film filters to modulate thevernier optical passband and maintain a substantially stable laseroutput power

401. The external cavity tunable laser with a laser output of 400,wherein the control circuitry synchronously modulates the vernieroptical passband and the laser electrical signal402. The external cavity tunable laser with a laser output of 401,wherein the laser output is a pulse signal and the synchronousmodulation varies the time duration of the signal pulse403. The external cavity tunable laser with a laser output of 400,wherein the laser temperature is controlled and the control circuitrysynchronously modulates the vernier optical passband and the lasertemperature404. The external cavity tunable laser with a laser output of 400,wherein the laser output is maintained at a constant wavelength405. A tunable laser generating a laser output comprising:

a solid state laser medium operative to generate coherent radiationthrough a first facet in response to an electrical signal;

a lens operative to collect the coherent radiation and generate acollimated light beam;

a reflective surface for reflecting the collimated beam back to the lensand the laser medium thereby creating an external laser cavity;

an optical filter with an optical thermally tunable transmission bandwithin the operative range of the laser medium and positioned within theexternal laser cavity between the reflective surface and lens, thetunable transmission band changing wavelength due to absorption of thecollimated beam

406. The tunable laser of 406 wherein the laser output power ismodulated by the absorption of the collimated beam407. The tunable laser of 406 wherein the optical linewidth of the laseroutput is increased by the absorption of the collimated beam500. A thermally tunable optical filter assembly comprising a first andsecond thermally conductive substrate;

a first and second thermo-optic filter region thermally isolated fromthe first and second substrate, the first and second filter regionshaving a first and second free spectral range respectively;

a temperature control assembly for controlling the temperature of thefirst and second filter region;

the first and second thermally conductive substrates bonded together tocreate a thermal isolation gap between the first and second filterregions

501. The thermally tunable optical filter assembly of 500, wherein thethermal isolation gap contains a vacuum502. The thermally tunable optical filter assembly of 500, wherein thethermal isolation gap contains a gas with absorption lines within anoptical transmission bandpass of the first thermo-optics filter region503. The thermally tunable optical filter assembly of 500, wherein thesurfaces of the first and second filter regions facing the thermalisolation gap create an etalon with an optical transmission bandpasswithin the optical transmission bandpass of the first and second filterregions504. The thermally tunable optical filter assembly of 500, wherein thesurfaces of the first and second filter regions facing the thermalisolation gap are non-parallel600. A method of tuning an external cavity laser comprising

Supplying a control signal to a laser gain medium to generate acontinuous wave operation laser output;

Setting a first operating condition of a tunable component within theexternal cavity to select a first optical feedback and a firstwavelength of laser operation;

Changing to a second operating condition of the tunable component withinthe external cavity to select a second optical feedback, the laser stilloperating at the first wavelength

Generating a pulse within the control signal to begin a secondwavelength of laser operation.

601. A method of tuning an external cavity laser of 600, wherein thepulse length is less than 10 microseconds602. A method of tuning an external cavity laser of 600, wherein thepulse is created by modulating the control signal by less than 50% ofits initial amplitude603. A method of tuning an external cavity laser of 600, wherein anintracavity optical mode is used to maintain laser operation at thefirst wavelength while changing the operating condition of the tunablecomponent604. A method of tuning an external cavity laser of 603, wherein theintracavity mode is created between facets of the gain medium605. A method of tuning an external cavity laser of 603, wherein theintracavity mode is created between a facet of the gain medium and anexternal cavity mirror606. A method of tuning an external cavity laser of 603, wherein theintracavity mode is created by the tunable component607. A method of tuning an external cavity laser of 600, wherein thetunable component is a grating, the operating condition is mechanicalposition and the optical feedback condition is a wavelength reflectionpeak.608. A method of tuning an external cavity laser of 600 wherein thetunable component is a thermo-optic filter, the operating condition istemperature, and the feedback condition is optical bandpass wavelength.

While various embodiments of the invention have been particularly shownand described, it will be understood by those skilled in the art thatvarious changes in form and details may be made therein withoutdeparting from the scope of the invention as defined by the appendedclaims.

What is claimed is:
 1. A tunable laser with a tunable laser output,comprising: a solid state laser medium having a first facet, a secondfacet, and an optical gain region, the second facet being a reflectiveend facet, the laser medium being operative to generate coherentradiation through the first facet in response to an electrical signal; areflective surface reflecting the coherent radiation back to the solidstate laser medium and the second facet; and an optical filterpositioned between the reflective surface and first facet, the opticalfilter having two surfaces and a thermo-optically tunable transmissionregion with associated thermally tunable optical transmission band, thecoherent radiation being incident on each surface of the optical filter,the optical filter being configured and operative to thermally tune thetunable laser output to a wavelength of laser operation and generate aspecularly reflected optical output from the coherent radiation, whereinthe specularly reflected optical output is directed away from the solidstate laser medium and forms substantially all of the tunable laseroutput.
 2. The tunable laser of claim 1, wherein a lens is positionedbetween the first facet and the optical filter and operative to collectthe coherent radiation and generate an optical beam.
 3. The tunablelaser of claim 2, wherein the optical beam is partially collimated oruncollimated when incident on the optical filter.
 4. The tunable laserof claim 3, wherein a surface of the optical filter is curved.
 5. Atunable laser with a tunable laser output, comprising: a solid statelaser medium operative to generate coherent radiation; and an opticalfilter having two surfaces and a thermo-optically tunable transmissionregion with associated thermally tunable optical transmission band, theoptical filter arranged to have the coherent radiation incident thereon,wherein the optical filter includes: a resistive heater for changing atemperature of the thermo-optically tunable transmission region, theresistive heater having a spatial location and patterned shape creatinga spatially non-uniform heating of the thermo-optically tunable regionsuch that an optical bandpass of the thermally tunable opticaltransmission band spatially varies in a direction orthogonal to adirection of propagation of the coherent radiation in the opticalfilter.
 6. The tunable laser of claim 5, wherein a lens is positionedbetween the first facet and the optical filter and operative to collectthe coherent radiation and generate an optical beam.
 7. The tunablelaser of claim 6, wherein the optical beam is partially collimated oruncollimated when incident on the optical filter.
 8. The tunable laserof claim 7, wherein a surface of the optical filter is curved.
 9. Thetunable laser of claim 7, wherein the spatially non-uniform heatingreduces optical bandpass spatial non-uniformity of the thermo-opticallytunable region resulting from absorption of the optical beam by theoptical filter.
 10. The tunable laser of claim 5, wherein a region ofthe optical filter is thermally isolated by a thermal isolationstructure and the spatially non-uniform heating reduces optical bandpassnon-uniformity of the thermo-optically tunable region resulting from thethermal isolation.
 11. The tunable laser of claim 5, wherein the heaterincludes a first heater and a second heater, the first and secondheaters having a separate electrical interconnect for independentlycontrolling electrical power to each heater.
 12. The tunable laser ofclaim 11, wherein the power to the second heater varies as a function ofthe optical power of the coherent radiation.
 13. The tunable laser ofclaim 11, wherein the optical filter is a first optical filter andfurther including a second optical filter, the first optical filter andsecond optical filter being etalons with different free spectral ranges,wherein the electrical power to the second heater changes a finesse ofthe first optical filter to change the power or wavelength of thecoherent radiation.